Thermochromic material

Abstract

A print on-demand color shift material which has high reflection, high saturation color and intrinsic, high impact color-shift produced by thermal printing or laser writing for use, for example in security laminates, tax and excise stamps, machine-readable features and banknote foils, and offering a rainbow of colors with full color shifting, producible in full manufacturing scale.

The invention claimed is: 
     
       1. A tunable multilayer optical film having at least two layers, at least one layer comprising a plurality of fluidizable particles, there being interstitial void space between the particles, the at least one layer having a first refractive index and a first optical thickness, the other of the at least two layers having a second refractive index and a second optical thickness, the first refractive index being different from the second refractive index. 
     
     
       2. The tunable multilayer optical film according to  claim 1 , wherein the fluidizable particles comprise a core-shell configuration. 
     
     
       3. The tunable multilayer optical film according to  claim 2 , wherein the fluidizable particles comprise more than 20% by mass of at least one layer. 
     
     
       4. The tunable multilayer optical film according to  claim 2 , wherein the layers are between 10 nm and 1000 nm in thickness. 
     
     
       5. The tunable multilayer optical film according to  claim 2 , wherein at least 80% of the particles are less than 200 nm in diameter. 
     
     
       6. The tunable multilayer optical film according to  claim 1 , wherein coating components occupy at least a portion of the interstitial void space. 
     
     
       7. The tunable multilayer optical film according to  claim 6 , wherein the multilayer optical film has an initial set of optical properties, wherein upon an application of energy, the multilayer optical film's optical properties change to a second set of optical properties. 
     
     
       8. The tunable multilayer optical film according to  claim 7 , wherein the energy is provided in the form of thermal or radiant energy. 
     
     
       9. The tunable multilayer optical film according to  claim 8 , wherein the initial set of optical properties differs from the second set of optical properties in at least the property of optical thickness. 
     
     
       10. The tunable multilayer optical film according to  claim 8 , wherein the initial set of optical properties differs from the second set of optical properties in at least the property of geometric thickness. 
     
     
       11. The tunable multilayer optical film according to  claim 7 , wherein upon the application of energy, the interstitial void space is at least partially eliminated and/or the fluidizable particles may at least partially fuse together or consolidate. 
     
     
       12. The tunable multilayer optical film according to  claim 11 , wherein upon the application of energy, a fluid formed from the fluidizable particles infiltrates a layer other than the layer comprising those fluidizable particles. 
     
     
       13. The tunable multilayer optical film according to  claim 7 , wherein the tunable multilayer optical film is fixed so that it is no longer tunable. 
     
     
       14. The tunable multilayer optical film according to  claim 7 , wherein the interstitial void space has been partially eliminated. 
     
     
       15. The tunable multilayer optical film according to  claim 7 , wherein the tunable multilayer optical film is comprised of alternating layers. 
     
     
       16. The tunable multilayer optical film according to  claim 7 , wherein the fluidizable particles are selected from the group of salts, organic or inorganic molecular compounds, polymers, ionomers, crystals and other compounds capable of becoming fluid at a transition temperature. 
     
     
       17. A security device comprising a tunable multilayer optical film according to  claim 1 . 
     
     
       18. The security device according to  claim 17 , wherein the tunable multilayer optical film is applied to an exterior surface of the security device. 
     
     
       19. The security device according to  claim 17 , wherein the tunable multilayer optical film is encased within the security device. 
     
     
       20. The security device according to  claim 17 , wherein the tunable multilayer optical film is encased within polycarbonate. 
     
     
       21. The security device according to  claim 17 , wherein the tunable multilayer optical film is encased within a security thread. 
     
     
       22. The security device according to  claim 17 , wherein the tunable multilayer optical film is transferred onto a layer of the security device. 
     
     
       23. The security device according to  claim 17 , comprising a pattern formed by an application of energy to the multilayer optical film. 
     
     
       24. The security device according to  claim 23 , wherein the pattern comprises a serial number, portrait, signature, finger print, date or other data. 
     
     
       25. The security device according to  claim 17 , wherein the security device is used for identifying an individual. 
     
     
       26. The security device according to  claim 17 , wherein the security device is a banknote. 
     
     
       27. A method for fabricating a tunable multilayer optical film according to  claim 1  comprising the steps of:
 a. preparing at least two dispersions for forming layers in the tunable multilayer optical film, at least one of the dispersions containing fluidizable particles; 
 b. coating a first dispersion onto a substrate so that the first dispersion forms a first layer; 
 c. coating a second dispersion onto the first layer to form a second layer; 
 d. coating each remaining dispersion onto a previous layer, which may be the same or different; and 
 e. drying and/or curing the layers, either individually, in groups and/or all together to form the tunable multilayer optical film. 
 
     
     
       28. The method according to  claim 27 , wherein the at least one of the dispersions containing fluidizable particles further comprises a coating component. 
     
     
       29. The method according to  claim 28 , further comprising the step of applying energy to the tunable multilayer optical film to change the optical properties of the tunable multilayer optical film. 
     
     
       30. The method according to  claim 29 , further comprising the step of applying a stimulus to the tunable multilayer optical film to fix the optical properties of the tunable multilayer optical film.